Installing & Using the LLVM Back-end

Installing

Apply the darcs patch linked above to GHC head. This will make some changes across GHC, with the bulk of the new code ending up in 'compiler/llvmGen'.

To build GHC you need to add one flags to build.mk, it is:

GhcEnableTablesNextToCode = NO

The LLVM code generator doesn't support at this time the TABLES_NEXT_TO_CODE optimisation due to limitations with LLVM. As long as its disabled the build system will detect this an automatically add in the LLVM backend.

LLVM

You will also need LLVM installed on your computer to use the back-end. If you wish to simply use an unregistered build of GHC, then the back-end should work with any standard LLVM version. If you wish to use a registered build of GHC however, then you may need to apply a patch to LLVM and build it yourself:

Using

Once GHC is built, you can trigger GHC to use the LLVM back-end with the -fllvm flag. There is also a new -ddump-llvm which will dump out the LLVM IR code generated (must be used in combination with the -fllvm flag. (or use the -keep-tmp-files flag).

ghc --info should also now report that it includes the llvm code generator.

The ​ghc-core tool also supports the llvm backend, and will display the generated assembly code for your platform.

Supported Platforms & Correctness

Linux x86-32/x86-64 are currently well supported. The back-end can pass the test suite and build a working version of GHC (bootstrap test).

Mac OS X 10.5 currently has a rather nasty bug with any dynamic lib calls (all libffi stuff) [due to the stack not being 16byte aligned when the calls are made as required by OSX ABI for the curious]. Test suite passes except for most the ffi tests.

Windows 32bit: The backend works for most things but no extensive testing or support yet.

Other platforms haven't been tested at all. As using the back-end with a registered build of GHC requires a modified version of LLVM, people wanting to try it out on those platforms will need to either make the needed changes to LLVM themselves, or use an unregistered build of GHC which will work with a vanilla install of LLVM. (A patch for LLVM for x86 is linked to below.)

Performance

(All done on linux/x86-32)

A quick summary of the results are that for the 'nofib' benchmark suite, the LLVM code generator was 3.8% slower than the NCG (the C code generator was 6.9% slower than the NCG). The DPH project includes a benchmark suite which I (David Terei) also ran and for this type of code using the LLVM back-end shortened the runtime by an average of 25% compared to the NCG. Also, while not included in my thesis paper as I ran out of time, I did do some benchmarking with the 'nobench' benchmark suite. It gave performance ratios for the back-ends of around:

Validate

The GHC patch has been validated to make sure it won't break anything. This is just compiling and running GHC normally but with the LLVM back-end code included. It doesn't actually test the LLVM code generator, just makes sure it hasn't broken the NCG or C code generator.